@Simon (betty02)
The easiest way to start is just by editing the original default theme supplied in the original Chameleon RC4 binary download. So for a first attempt, you could load up each graphic from the default theme, leaving it at the same size, and replace with your image. Then re-save as a .png with a transparent background.
This way you won't have to worry about editing he theme.plist at this stage. It's only when you add a background image will the resolution become important.
With regard to a scroll bar, you have to remember that Chameleon is primarily a bootloader and the GUI is a secondary feature and the graphical features are limited. The current implementation of a scroll is the have a default number of devices set in the theme.plist and when you have more than the default, then an icon will be drawn to allow you to see the next/previous devices.
@Rocksteady
Thanks for the kind words